Fungicide resistance management is too important to be left to chance.

Fruit growers will have new fungicides this year to help manage diseases as diverse as leaf spot in cherries, scab in apples, and brown rot and scab on peaches.

The new materials are so-called second-­generation succinate dehydrogenase inhibitors (SDHI), similar in action to boscalid, one of the components of Pristine (the first in this new generation, registered in 2003), and they are being sold in a similar manner. They come in packaged premixtures, in which the new SDHI component is combined with an older fungicide with a different Fungicide Resistance Action Committee (FRAC) code and different mode of action.

These premixtures are designed to prevent development of resistant disease organisms, which can and do destroy the value of good fungicides, Dr. George Sundin, Michigan State University plant pathologist, explained during the Great Lakes Fruit, Vegetable, and Farm Market Expo in Grand Rapids, Michigan, in December. Packaged premixtures currently registered or soon to be available within the next year are:

Fontelis, a new product coming from DuPont contains the new active ingredient penthiopyrad, and will be sold mixed with other fungicides. BASF’s Pristine is a combination of pyraclostrobin and boscalid (SDHI, FRAC Group 7).

The companies announced last year that they were introducing or had introduced the new fungicides and, in the case of Bayer CropSciences, announced that they would market the products with different names depending upon the combination of fungicide in the products. Luna Sensation 500 SC is a mixture of fluopyram and trifloxystrobin, and Luna Experience is fluopyram plus tebuconazole. Luna Tranquility is another mixture being tested on grapes. Luna Sensation has been tested on cherries and apples for leaf spot and scab.

University scientists have recommended for some years that new fungicides with narrow modes of action be tank-mixed with broad-spectrum protectant fungicides to which disease organisms have a low chance of developing resistance, according to Sundin. These protectants include the inorganic materials copper and sulfur; the dithiocarbamates maneb, mancozeb, and thiram; the chloronitrile chlorothalonil; and the dicarboximide phthalimide captan.

Andy Wyenandt, a plant pathologist at Rutgers ­University in New Jersey, says protectant fungicides are contact fungicides, meaning they must be present on the leaf surface prior to the arrival of the fungus and must then come into direct contact with the fungus.

“Protectant fungicides can be redistributed on the leaf surface with rainfall or overhead irrigation, but can also be washed off by too much of either,” he warned. “With protectant fungicides, any new growth is unprotected until the next protectant fungicide is applied. In other words, protectant fungicides are not systemic and do not have translaminar activity like some of the newer chemistries. Protectant fungicides should be tank-mixed with ­fungicides with higher risks for resistance development.”

Generally, plant pathologists recommend using protectant materials early in the season to prevent buildup of disease and saving the new fungicides for use later, when fruit is most vulnerable and disease pressure is strong.

They usually recommend growers include a protectant fungicide in every spray application. And they recommend rotating fungicides, paying attention to FRAC groups to avoid cross-resistance.

It is also pretty much agreed that growers should not wait for disease to be observed in their orchards, as they might when using IPM practices against insects. Fungicide sprays should be applied early, before disease symptoms appear, and based primarily on weather conditions that govern risk of disease development in the orchard.

This wise advice developed from experience. Fungicide resistance has been a problem since the first of these narrow (site-specific) mode of action fungicides, Benlate (benomyl), was introduced in 1972.

Sundin suggested that the chemical companies, weary of seeing their products become ineffective and less marketable because of resistance, are now marketing their new products in mixtures designed to reduce resistance development. They are, in effect, insurance policies against the potential rapid development of resistance. The new products are translaminar or locally ­systemic—they move up and down on the leaf surface or inside the plant—and they have curative as well as ­preventive modes of action.

Lessons from Benlate

Dr. David Ritchie, plant pathologist at North Carolina State University, told stone fruit growers at the Great Lakes Expo that, back in 1972, the introduction of the new fungicide Benlate changed what growers expected a fungicide to do. But it also taught them unexpected lessons on dealing with emergence of fungicide-resistant diseases.

Unlike the protectant fungicides in use before then, benomyl provided control of multiple diseases and was taken up and moved systemically within the plant, meaning that wash-off was less important and that disease control could be achieved for unsprayed parts of the plant, Ritchie said. Furthermore, it had protective and postinfection activity, and was highly effective at ounces rather than the pounds of product per acre required for other fungicides such as sulfur and ­captan. But another characteristic became its Achilles’ heel: benomyl had a site-specific mode of action.

Following three consecutive years of exclusive use of Benlate on cherries from 1973 to 1975, brown rot control failed, he said. It was found that strains of the fungus had become resistant to the fungicide and that there was cross resistance to other fungicides with a similar mode of action. There was the realization that, like the earlier experience entomologists had with insect resistance to DDT, plant pathologists would now have to deal with the prospect of pathogens becoming resistant to fungicides.

After benomyl, chemical companies and growers became involved in a continual search for new fungicides that would replace those that became ineffective after repeated spray applications.

Benomyl was followed by the sterol biosynthesis inhibitors (SIs), which also include the DMIs Orbit (propiconazole), Indar (fenbuconazole), and Elite (tebuconazole), and now their generics.

Then came the dicarboximides like Rovral (iprodione) and Ronilan (vinclozolin). These were followed in the 1990s by the strobilurins [Abound (azoxystrobin), Flint (trifloxystrobin), and Gem (trifloxystrobin)]. All of these were prone to resistance development.

After some years of discussion about whether fungicides of different modes of action should be tank mixtures or alternated sprays, or a combination, along came Pristine, the first of the packaged premixtures.

After growing up on a Michigan dairy farm, Richard Lehnert began writing about farming in 1962, while still a junior studying journalism at Michigan State University. He worked at newspapers for a year before joining the staff of Michigan Farmer, where he spent 26 years, the last 15 as chief editor. He joined the staff of Good Fruit Grower in 2010.
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